1. Field of the Invention
The present invention relates to deposition of metal nitride thin films. In particular, the invention concerns methods of growing metal nitride thin films by Atomic Layer Deposition (“ALD”).
2. Description of the Related Art
Integrated circuits contain interconnects, which are usually made of aluminum (Al) or copper (Cu). Cu is particularly prone to diffusion or electromigration into surrounding materials, which may adversely affect the electrical properties of the IC and cause active components to malfunction. Diffusion of metals from interconnects into active parts of the device can be prevented with an electrically conductive diffusion barrier layer. Preferred diffusion barriers are, e.g., amorphous and stoichiometric transition metal nitrides, such as TiN, TaN and WN. The nitrides can also be non-stoichiometric if nitrogen occupies lattice interstitial sites. In addition to barrier applications, metal nitride films are also used in metal gates (mid gap films) and MIM/MIS capacitors as bottom and/or top electrodes.
For certain semiconductor device applications, stoichiometric (i.e., metal-to-nitrogen ratio equal to one) metal nitride films are preferred over non-stoichiometric metal nitride films. A stoichiometric metal nitride films has a higher electrical conductivity (lower electrical resistivity) than a non-stoichiometric metal nitride film, making it ideal for use as a diffusion barrier. However, stoichiometric metal nitride films have been found difficult to form by atomic layer deposition (ALD).
ALD, sometimes called atomic layer epitaxy (ALE), is a self-limiting process, whereby alternating and sequential pulses of reaction precursors are provided to deposit no more than one monolayer of material per deposition cycle. The deposition conditions and precursors are selected to ensure self-saturating reactions, such that an adsorbed layer in one pulse leaves a surface termination that is non-reactive with the additional gas phase reactants of the same pulse. A subsequent pulse of different reactants reacts with the previous termination to produce the desired material and enable continued deposition. Thus, each cycle of alternated pulses leaves no more than about one molecular layer of the desired material. The principles of ALD type processes have been presented by T. Suntola, e.g. in the Handbook of Crystal Growth 3, Thin Films and Epitaxy, Part B: Growth Mechanisms and Dynamics, Chapter 14, Atomic Layer Epitaxy, pp. 601-663, Elsevier Science B.V. 1994, the disclosure of which is incorporated herein by reference.
In a typical ALD process, one deposition cycle comprises exposing the substrate to a first reactant, such as a metal precursor, removing unreacted first reactant and reaction by-products, if any, from the reaction chamber, exposing the substrate to a second reactant, followed by a second removal step. Typically, halide reactants, such as TiCl4 and HfCl4, are used as metal precursors in ALD deposition because those reactants are inexpensive and relatively stable, but at the same time reactive towards different types of surface groups. Where formation of metal nitride thin films is desired, ammonia (NH3) is typically used as a second precursor, though other nitrogen-containing compounds may also be used.
Surplus chemicals and reaction by-products, if any, are removed from the reaction chamber before the next reactive chemical pulse is introduced into the chamber. The separation of reactants by inert gas prevents gas-phase reactions between reactants and enables self-saturating surface reactions. As a result, ALD growth generally does not require strict temperature control of the substrate or precise dosage control of the reactants. Undesired gaseous molecules can be effectively expelled from the reaction chamber by maintaining a substantially high gas flow rate of a purge gas. The purge gas directs the unreacted molecules toward the vacuum pump used for maintaining a suitable pressure in the reaction chamber. ALD advantageously provides accurate control of the composition, thickness and uniformity for thin films.
Methods of forming metal nitride layers by ALD are know in the art. For example, U.S. Pat. No. 6,863,727 to Elers et al., issued Mar. 8, 2005, the entire disclosure of which is incorporated herein by reference, teaches a “3-step” ALD method of forming a metal nitride film comprising alternately and sequentially contacting a substrate with a vapor-phase pulse of a metal source chemical, a boron-containing reducing agent and a nitrogen source chemical. However, metal nitride films formed via the 3-step method have a tendency to yield non-stoichiometric metal nitride films. Alternative methods, which employ enhanced reducing agents, have faced similar problems.
Additionally, metal nitride films are reactive towards oxygen and other oxidizing agents. This is problematic because metal nitride films formed according to methods available in the art may oxidize prior to processing steps that follow formation of the films, for example during transport to another processing chamber. Oxidized metal nitride films are undesirable because they impede contact between the metal nitride film and overlying layers in typical semiconductor device structures. If the oxide layers are substantially thick, the electrical properties of the metal nitride films may be adversely affected.
Accordingly, there is a need in the art for ALD methods of forming stoichiometric and passivated metal nitride films, wherein the surfaces of the films are resistant to oxidation.